The present invention relates to mobile radio communication systems using code division multiple access (CDMA) techniques. It lies within the receivers used in the fixed stations of these systems and operating a coherent demodulation of spread spectrum signals originating from a set of radio terminals.
Coherent demodulation requires various parameters representing the propagation channel between the transmitter and the receiver. Some of these parameters vary relatively slowly and can be estimated by statistical survey procedures. Such is the case for example for the delays assigned to the multiple propagation paths in the conventional rake receiver. The delays specific to the various paths can be updated at fairly low frequency, for example of the order of about a hundred milliseconds. Other parameters have abrupt variations, on the scale of the duration of an information symbol, which are due to the phenomenon of fading. Such is the case in particular for the instantaneous amplitudes of reception of the symbols following the propagation paths taken into consideration, which are necessary for coherent demodulation. These instantaneous amplitudes are complex amplitudes, manifesting the attenuation and the phase shift which are undergone at each instant along the paths. In general, these complex amplitudes are estimated from symbols known a priori, or pilot symbols, interspersed among the information symbols transmitted so as to allow coherent demodulation.
It is known that CDMA receivers are affected by the near-far problem according to which transmission from mobiles near a base station tends to mask that from distant mobiles. The reason is that all these mobiles share the same uplink frequency at the same time, the distinction between them resulting from the quasi-orthogonality of the spreading codes. Transmission power controls procedures are implemented in order to limit the impact of this problem, but nonetheless the base station does not receive with the same power the signals transmitted by the various mobiles, in particular when the spreading factors of the various channels are not the same or when a mobile is moving fast, which may bias the power control algorithm.
In general, channel estimation methods make the assumption that the noise present on a channel is white noise. This assumption is correct in the case of thermal noise, although not for the noise generated by the other transmitters. The estimation methods then generate errors which are especially appreciable when the power received on a channel dominates that received on one or more other channels.
An object of the present invention is to make the estimates of superposed CDMA channels more reliable in order to improve the performance of the receivers.